Without limiting the scope of the invention, the background is described in reference to the wafer cleaning process steps in an integrated circuit production facility. Semiconductor devices are typically fabricated on a circular semiconductor substrate called a wafer. The circuitry for the electronic devices is fabricated on a wafer of semiconductor material using photolithography and vapor phase deposition techniques, and also selective etching techniques. Many electronic devices are produced on a single wafer. After the individual devices are completely fabricated, the wafer is cut into the individual die.
The wafers used in the fabrication of semiconductor devices are typically circular and quite thin. One problem which arises in producing circuitry on the circular wafers is that particulate matter which accumulates on the outer edge and the outer circumference of the wafer, in areas where no circuitry is defined, migrates into the areas where circuitry is being defined. Particles left on wafers will be redistributed during wafer processing and will move onto active die in the area of the wafer that is used for production. These particles cause defects in the circuitry being defined on the wafer. As device geometries continue to shrink, these particles will become larger compared with the device geometries and the defects will correspondingly be more critical. These defects result in nonfunctional electronic devices, which reduce the unit yield per wafer and correspondingly increase the cost of production per unit.
Known prior art cleaning systems use a cleaning solution in a tank coupled with a megasonic transducer to remove particulate matter from top and bottom interior surfaces of the wafers. These known systems fail to adequately address the problem of removing particulate matter from the outer edge and outer circumference of the wafers. In known systems, the wafers are placed in the tank of cleaning chemicals and the tank is excited by energy radiating from the megasonic transducer, which increases the rate of particle removal. Because the wafers are placed in the tank in a vertical orientation, some parts of the wafer are farther away from the megasonic energy source than others. Typically the transducer is located at the bottom of the tank and the energy radiates from it. This results in a nonuniform cleaning rate from the bottom of the wafers to the top. As wafers increase in size to accommodate larger circuits and increasing integration, these effects will increase in significance. Known cleaning systems correct for this nonuniform effect by using more processing time than that required at the areas closer to the transducer in order to extend the proper cleaning results to those areas farther away. In spite of this, the wafers are cleaned nonuniformly.
Additional particle problems arise because the wafers are stored in carriers or boats. In the megasonic processing tanks of the prior art, the wafers are placed into the tank while residing in a wafer boat. Each wafer rests within grooves that separate the wafers and prevent them from colliding as they are moved. The wafers are vertically oriented within the boat. The sides and a small portion of the bottoms of the wafers are therefore contacting the boat. These surfaces are not cleaned efficiently by the megasonic processing tanks of the prior art, because they are essentially dead spots. Particles trapped in these places are not effectively removed because the megasonic energy is partially blocked and because the particles are trapped between the wafers and the boat. Another area of the wafers that is not well cleaned by the cleaner configurations of the prior art is the top edge of the wafers. The tops edges of the wafers facing away from the transducer do not receive direct line of sight megasonic energy and therefore are not cleaned as effectively as the bottom edge. Particles left in these places will migrate to the active areas of the wafer and become particle defects in the wafer when the wafer is further processed.
Some known systems move wafers about in the tanks during cleaning but none address the problem associated with nonuniform exposure to the megasonic energy source or the spaces between the wafer edge and the boats. Prior art approaches include moving the boats from side to side with a mechanism in the tank or using a robot arm to move the carrier during processing. These approaches risk additional particle contamination within the tank by introducing additional surfaces into the tank. Further, using the robot arms in automated process flows to move the wafers and boat could require that the robot arm move the wafers during the entire megasonic processing cycle, which ties up the robot arm and decreases overall process flow throughput. The robot arm is typically used for many tasks, including moving the wafers into the megasonic tank. If the robot arm is required to stay in place throughout the megasonic cycle, the overall throughput of the system is decreased. Other tasks for the robot arm are now required to be deferred until after the megasonic processing is completed.
The nonuniform results of known prior art megasonic processes are detectable with current technology wafers of 4 or 6 inches, and will become even more pronounced with 8 inch or larger wafers as the standard. Larger wafers are being contemplated, which will make particle defects more critical still. The invention of this application addresses the nonuniform cleaning rates and the extended processing times for obtaining acceptable results with prior art megasonic cleaning and etching systems, and the particles left in place in prior art wafer cleaning and etching systems.